In recent years, environmental problems have been on the rise around the globe, and thus methods for reducing fuel consumption in order to cope with such problems in entire industries have been sought. To reduce fuel consumption, solutions proposed in the vehicle industry include improving efficiency of vehicle engines and weight reduction of vehicles. Reducing the weight of the vehicle may be a measure capable of increasing fuel efficiency of the vehicle. However, if the weight of the vehicle is reduced, there may occur a problem in that the strength and durability required in vehicles may not be satisfied. Therefore, a solution thereof becomes a major goal of the vehicle industry.
Therefore, in the vehicle industry, various environmentally-friendly vehicles have been developed with an object of reducing a discharge amount of carbon dioxide to 95 g/km, which is 27% of the current amount thereof, by 2021 based on European regulations. Furthermore, vehicle makers strive to develop technology to downsize and improve fuel economy in order to meet 54.5 mpg (23.2 km/1) which is the required value of the corporate average fuel economy (CAFE) in the USA by 2025.
Generally, in order to cope with an increase in the number of parts or an increase in weight, the weight of a material is reduced. In this case, as a weight reduction method, a heat treatment technology for implementing high strength of the material or curing a material surface is frequently used. Furthermore, to cope with complicated part shapes, precise joining, low-distortion welding, and low-distortion heat-treatment technologies are used. In addition, technology for reducing distortion caused by heat treatment, and noise reduction and dust-removing technologies are used.
For example, a high-performance and high-efficiency technology for engines and transmissions for maximizing fuel economy of vehicles has been developed, and this technology includes increased number of gears, a novel concept driveaway device, high-efficiency two-pump systems, fusion hybrid technology, technologies related to an automatic/manual fusion transmission, a hybrid transmission, and the like.
An alloy steel used in technology relating to engines and transmissions may be used in parts of the engine, carriers of the transmission, gears, shafts, synchronizer hubs, or the like, and a use ratio of the alloy steel corresponds to 32 to 40% based on the weight of the engine and about 58 to 62 wt % based on the weight of the transmission. For example, in the gears of the transmission and the like, development of highly strengthened and highly durable materials meeting the requirements of weight reduction and downsizing have been continuously required. However, a technology relating to downsizing or improvement of fuel efficiency has problems in that a load applied to the parts of the engines is increased, quality of the parts is reduced, and durability life is reduced due to burning, friction, abrasion, and the like.
Generally, the gears of the transmission of the vehicle are parts that directly transfer engine power to a differential system and effectively transfer rotation or power between two or more shafts so that engine power is attuned to a driving state of the vehicle, and bending stress and contact stress are simultaneously received. In the gears, when durability of the material is insufficient, fatigue failure (tooth breakage) due to a lack of bending fatigue strength and fatigue damage (pitting) due to a lack of contact fatigue strength frequently occur. Therefore, in the gears, physical properties such as hardness, strength, toughness, fatigue strength, and fatigue life are required.
As an alternative to the aforementioned requirements, currently, a carburized steel such as SCM820PRH including 0.17 to 0.23 wt % of carbon (C), 0.5 to 0.7 wt % of silicon (Si), 0.45 to 0.75 wt % of manganese (Mn), 1.95 to 2.25 wt % of chromium (Cr), 0.015 to 0.035 wt % of molybdenum (Mo), 0.0015 wt % of oxygen (O2), and the like is used. However, this carburized steel has a problem in that tooth breakage and pitting easily occur.